Apparatus and method for concentrating solutions



Jan. 9, 1968 I A. N. CHIRICO 3,362,457

APPARATUS AND METHOD FOR CONCENTRATING SOLUTIONS Filed March 18, 1966 POLISH ING uaswaqwoo omuwouva CEINTRIFU E ATTORNEYS United States Patent 3,362,457 APPARATUS AND METHOD FOR CUNCENTRATING SOLUTKQNS Anthony N. (Ihirico, Naperville, 11]., assignor to Chicago Bridge & Iron Company, Oak Brook, Hinsdale, 111., a corporation of Illinois Filed Mar. 18, 1966, Ser. No. 535,578 4 Claims. (Cl. 159-17) This invention relates to a method for concentrating a solution and separating crystals therefrom. In its more specific aspect, this invention relates to a method for concentrating a solution having two or more chemical solutes, crystallizing at least one solute, and separating the crystals from the remaining concentrated solution. The invention is described hereinbelow in detail with reference to electrolytic caustic soda, but it should be understood that the invention is equally applicable to other solutions having chemically different constituents.

In the manufacture of electrolytic caustic soda, the brine charge is partly converted through electrolysis into caustic soda. The liquor withdrawn from the electrolytic cells typically comprises about 8 to 12% sodium hydroxide, 13 to 17% sodium chloride, and the balance Water. In commercial practice, the caustic soda is separated from the salt by concentrating the cell liquor in a multiple effect evaporator system to crystallize most of the salt, which is then separated from the mother liquor.

In conventional multiple effect evaporator systems for concentrating caustic soda solutions, the crystallizer effects in the system are in order of decreasing operating pressure and backward feed is utilized. In a typical system, the first effect is operated at 125 to 150 F. and at subatmospheric pressure, whereas the last effect is operated at a temperature of approximately 300 F. and about atmospheric pressure or slightly above. Accordingly, the

feed liquor is introduced to the last effect of the system for vaporization and concentration and ultimately discharged from the first effect at about a 50% caustic soda concentration and at an elevated temperature of typically about 300 F.

Ordinary construction metals cannot be employed in much of the evaporator system because the hot, concentrated caustic corrodes the metal. Moreover, linings cannot be used because of the elevated temperatures. In view of this, it is necessary to employ exotic metals such as nickel and cupro-nickel to prevent corrosion.

This invention has as its purpose to provide a method for concentrating a solution in a multiple effect evaporator system requiring a reduced number of crystallizer evaporator effects and obviating the necessity for exotic metals, thereby realizing substantial economies. Reference is now made herein to the specific description and to the accompanying drawing which is a diagrammatic representation of the present invention for carrying out the improved process.

In referring to the drawing, there is now described the concentration of a solution comprising caustic soda and sodium chloride as obtained from diaphragm electrolytic cells. The liquor passes from the electrolytic cell (not shown) through a suitable supply line 10 to a recirculation system for the evaporator-crystallizer 12 of conventional forced circulation design.

The recirculation system for the crystallizer effect comprises interconnected piping including outlet line 14, a recirculating pump 16, line 18 leading from the pump to a shell-and-tube heat exchanger 20 and a return inlet line 22. Steam is supplied to the heating system of evaporatorcrystallizer effect 12 from a suitable boiler (not shown) by line 24 to a thermo-compressor 26 and then through branch line 28 to heat exchanger 20. The motive steam from the boiler is passed to the thermo-compressor at very high pressure where it expands and leaves the compressor at a lower pressure. Steam passing through the thermo-compressor aspirates vapors or steam from the vapor chamber of crystallizer effect 12 through line 30. A portion of the aspirated vapor is recirculated to the heat exchanger 20 via line 28, and the remainder of the vapor is passed through line 31. Condensate from the heat exchanger may be returned to the boiler through line 32.

The hot feed solution enters the recirculation system of the first effect 12 through line 14 where it admixes with relatively more concentrated liquor which may contain some salt crystals from the first effect. The feed liquor is further heated by indirect heat exchange on passage through heat exchanger 20 and enters the first effect 12 through line 22 at a relatively high temperature. The hot feed liquor is forced under pressure to the first effect, and under the operating conditions employed, the solution spontaneously adiabatically cools to the boiling temperature corresponding to the pressure existing in the vessel. The crystallizer effect is desirably operated at a pressure of about 20 to 30 p.s.i.g. The resulting cooling and evaporation of some of the solvent will supersaturate the solution with respect to the sodium chloride, and sodium chloride crystals are produced which may serve as nuclei or seeds to effect growth in the subsequent efiect of the evaporator system. This stage of the process is particularly important in that controlled nucleation or seed generation affects the size and habit of the crystals developed in the second evaporator effect, which is important in order to achieve optimum crystals and to improve the plant efiiciently as affected by the separation of the crystallized solute from the mother liquor. The vapors generated in the first effect 12 are drawn through branch line 31 to supply heat by indirect heat exchange to one or more of the operations described hereinbelow in greater detail.

Magma comprising a slurry of sodium chloride crystals in mother liquor is passed from the first effect 12 via line 36 to the second evaporator effect 38. Circulation pump 40 employed in line 36 facilitates the flow of slurry.

Similarly, second effect 38 has a recirculation system comprising outlet line 42, recirculation pump 44, line 45 leading from the pump to shell-and-tube heat exchanger 48 and return line 50 for introducing the liquor to the evaporator effect. Heat may be supplied to heat exchanger 48 from the vapors generated in first effect 12 and introduced to the heat exchanger through lines 31 and 52. Condensate from heat exchanger 48 may be returned to the boiler by line 54.

The second effect 38 of the evaporator system is operated under subatmospheric pressure and at a relatively low temperature to produce a concentrated solution of sodium hydroxide and to crystallize most of the sodium chloride. The second effect is preferably operated at a pressure of not over 25 millimeters of mercury absolute and at a temperature of from to F. Under these operating conditions, adiabatic cooling occurs thereby resulting in a supersaturated solution with respect to the salt to produce further crystal growth. The seed crystals 2 are grown to a size and quality that facilitates the efficient separation from the mother liquor. At this stage of the process, the desired concentration of the solution with respect to the sodium hydroxide and the desired crystallization of most of the sodium chloride are achieved. Accordingly, the mother liquor produced in the second effect comprises not less than 45% solution of sodium hydroxide, and more preferably not less than 48% sodium hydroxide. The vapors generated upon the evaporation of water from the liquor are withdrawn from the vapor zone of the evaporator effect 38 through line 56 to barometric condenser 58. The condensate from the condenser 58 is withdrawn through line 59 to heat exchanger 60.

The sodium chloride crystals which precipitate to the bottom of the vessel 38 are withdrawn therefrom through line 61, having pump 62, to cyclone separator 64 which separates the smaller crystals from the larger thereby thickening the slurry. The salt crystals are withdrawn from the cyclone and passed to a suitable centrifuge 66 for further separation of the salt from the mother liquor and the salt may be recovered and returned to the electrolytic cell for further processing in the manufacture of caustic soda. The salt recovered first may be washed with feed liquor withdrawn from line through line 67 and directed to the centrifuge to remove entrained caustic. The relatively clear mother liquor and wash liquor recovered from the centrifuge, and containing a very low percentage of sodium chloride, is Withdrawn through line 68 and may be redirected to the cyclone by a line 70 to further wash the crystals thereby removing entrained sodium hydroxide, or may be redirected via line 72 to the second exect 38. The salt is recovered from the centrifuge through line 73.

The relatively clear mother liquor, which may contain a very low percentage of sodium chloride, is withdrawn from second evaporator effect 38 through line 74 and is introduced to evaporating chamber 78 which comprises a flash vessel. The evaporating chamber 78 is operated under subatmospheric conditions, and preferably at a pressure lower than the pressure in the second effect 38, and further at a temperature somewhat lower than the temperature in the second effect whereby flash vaporization occurs. Steam from branch line 31 is passed through line 80 and booster 81 and then through aspirator 82 which leads to the barometric condenser 58. Line 83 leading from the evaporating chamber 78 opens to booster 81 and steam passing therethrough acts as an aspirator to create a vacuum in the evaporating chamber. The vapors generated in the evaporating chamber are passed by line 83 to booster 81 and aspirator 82 to the barometric condenser 58 and the condensate reprocessed.

The substantially crystal-free liquor is withdrawn from the evaporating chamber '78 through line 86 to slurry tank 88. Salt crystals in the liquor settle out under gravity and are withdrawn from the conical portion by line 90 having recirculation pump 92 for return to the cyclone through line 60. Relatively clear mother liquor containing a very low percentage of sodium chloride is withdrawn from the top portion of slurry tank 88 through liquor discharge line 94 having recirculation pump 96. The liquor discharge line is passed to a suitable recovery system such as polishing system 98 for further purification and recovery of the caustic soda product. Here, any fines of sodium chloride crystals are removed and returned to slurry tank 88 by line 99. Where desired, an appropriate outlet means may be provided in slurry tank 88 intermediate the outlet means 90 and 94 for withdrawing liquor containing a low percentage of sodium chloride through line 100, having recirculation pump 102, for recirculating the liquor for further treatment in the slurry tank. The liquor withdrawn through line 100 is directed to heat exchanger 60 where it is cooled by indirect heat exchange by the condensate from barometric condenser 58 thereby facilitating further separation of the sodium chloride from the caustic solution.

In the operation of the evaporating system in concentrating electrolytic caustic cell liquor comprising, for example, about 11.5% caustic soda, 16.33% sodium chloride and the balance water, the feed liquor from the electrolytic cell which typically is at a temperature of about 180 F. is fed from line 10 to recirculation line 14 of the first evaporator effect 12. This effect is operated at about 25 p.s.i.g. and at a relatively elevated temperature of about 267 F. Motive steam is passed through line 24 at a rate of 38,000 pounds per hour and at 600 p.s.i.g. to the thermo-compressor 26. The steam expands and leaves the compressor at 140 p.s.i.g. In the operation, 31,000 pounds of steam is aspirated from the crystallizer effect 12. Of this, 7,000 pounds is recycled to the heat exchanger 20 thereby making a total of 45,000 pounds of steam to the heat exchanger, and the remaining 24,000 pounds is passed through lines 31 and 52 to supply heat to heat exchanger 48 and line to booster 81. The composition of slurry withdrawn through outlet line 36 comprises about 21% caustic soda, 15% sodium chloride and the balance water. The slurry is passed to the second effect 38 which is operated at a pressure of 22 millimeters of mercury absolute and at a relatively low temperature of about F. whereby flash vaporization occurs resulting in additional crystallization of sodium chloride and concentration of the solution with respect to the sodium hydroxide. The salt crystals in the liquor settle out under gravity and are withdrawn from the bottom of the effect through line 61 to the cyclone 64 and centrifuge 66 for recovery of this product. The relatively clear mother liquor comprising about 48.5% sodium hydroxide and 2.5% sodium chloride is withdrawn from the second effect through line 74 and directed to evaporator chamber 78. Here further adiabatic cooling and concentration occurs, the resulting solution comprising about 49.5% sodium hydroxide. This solution, comprising about 49.5% caustic soda and 2 to 3% sodium chloride, is passed to slurry tank 88 and then to polishing units 98 where the solution is cooled to about 80 F. to cause further crystallization of salt thereby resulting in a solution containing about 1% salt solution.

It will be observed that by reason of my invention, concentration and recovery of the product are accomplished With fewer crystallizer evaporator effects than that required in conventional operations, which further reduces the utilities required. In addition, because the liquor is concentrated in a vessel operated at low temperatures and pressures, the need for expensive, exotic metals is eliminated. As a result, substantial economy is achieved.

What is claimed is:

1. A method for recovering a first solute from liquor and recovering liquor relatively concentrated with respect to a second solute, comprising:

(a) introducing the feed liquor to the recirculation system of a first evaporator crystallizer effect, and heating said feed liquor by indirect heat exchange, and passing the resulting heated feed liquor to said first effect, said first effect maintained under a pressure greater than atmospheric and at an elevated temperature, whereby vaporization of the liquor and crystallization of the first solute occurs resulting in liquor relatively more concentrated than said feed liquor with respect to the second solute;

(b) passing steam through a thermo-compressor communicating with said first effect to aspirate vapors generated in said first effect, and recirculating a portion of said vapors with said steam to heat said feed liquor in indirect heat exchange in said first mentioned heat exchanger;

(c) passing the resulting concentrated liquor from said first effect to a second evaporator crystallizer efiect, said second effect maintained at subatmospheric pressure and at a temperature relatively lower than said first effect and heated at least in part from vapors from said first effect by indirect heat exchange, whereby vaporization of liquor and crystallization of the first solute occurs resulting in further concentration of said liquor with respect to the second solute; and

(d) separating the crystals produced from said concentrated liquor.

2. A method according to claim 1 wherein vapors generated from said first effect are passed through an aspirating means communicating with said evaporating chamber whereby vapors generated in said evaporating chamber are aspirated therefrom.

3. A method for recovering sodium chloride from relaconduit means establishing communication between tively dilute caustic soda solution containing sodium chlosaid effects; ride and recovering relatively pure and concentrated (b) recirculation means for each of said effects includcaustic soda solution comprising: ing a heat exchanger whereby liquor withdrawn from (a) introducing the feed liquor to the recirculation sys- 5 the bottom portion of said efiect is passed through tern of a first evaporator crystallizer effect and heatsaid heat exchanger and returned to said effect; ing the feed liquor by indirect heat exchange, and (c) a feed liquor inlet means communicating with said passing the resulting heated feed liquor to said first recirculating means for said first effect prior to said effect, said first effect being maintained under a presheat exchanger; sure of from 20 to 30 p.s.i.g. and at an elevated 10 (d) thermo-compressor means communicating with temperature whereby vaporization of the liquor and said first effect and with said heat exchanger for crystallization of sodium chloride occurs; said first effect whereby steam passed through said (b) passing steam through a thermo-compressor comthermo-compressor means aspirates and compresses municating with said first effect to aspirate vapors vapors generated in said first effect; generated in said first effect, and recirculating a por- (e) conduit means for passing at least a portion of tion of said vapors with said steam to heat said feed said vapors generated from said first effect to the heat liquor on indirect heat exchange; exchanger of said second effect;

(c) passing the resulting concentrated liquor from said (f) first outlet means located in the bottom portion of first effect to a second evaporator crystallizer effect, said second effect for withdrawing crystal slurry said second effect maintained under a pressure of not therefrom; more than millimeters of mercury absolute and (g) second outlet means located in the bottom portion at a temperature relatively lower than said first effect of said second effect but above said first outlet means and heated at least in part from vapors from said for withdrawing concentrated liquor therefrom; first effect by indirect heat exchange, whereby vapor- (h) an evaporating chamber in communication with ization of the liquor and crystallization of sodium 25 said second outlet means of said second effect; chloride occurs resulting in further concentration of (i) aspirator means in communication with said evaposaid liquor; rating chamber and with said vapor outlet means of (d) passing a slurry of said crystals from said second said first effect for vacuumizing said evaporating effect to a recovery means and separating said crystals chamber; and from said slurry; (j) said evaporating chamber discharging into means (e) passing the resulting concentrated liquor from said for recovering concentrated liquor substantially free second effect to an evaporating chamber maintained from crystals of said first solute. under a subatmospheric pressure whereby vaporization of liquor and crystallization of the sodium chlo- References Cited ride occurs resulting in further concentration of said 5 UNITED STATES PATENTS liquor;

(f) passing at least a portion of said vapors generated fig Trump 159 17 X 34 Ebner 23-1 in said first effect to an aspirating means In com- 2148 579 2/1939 R h 154 eic 47 X mumcatron with said evaporating chamber to aspirate 2 631 926 3/1953 Eckstrom 159 17 X the vapors generated therein; and

(g) separating the crystals produced from the resulting FOREIGN PATENTS concentrated liquor from the evaporating chamber 1387120 12/1964 France. and recovering relatively pure concentrated liquor. 1:111:149 7/1961 Germany 4. In a multiple effect evaporator system for removing a first solute from liquor and recovering liquor relatively concentrated with respect to a second solute, comprising: (a) first and second evaporator crystallizer effects, and

NORMAN YUDKOFF, Primary Examiner.

J. SOFER, Assistant Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,362,457 January 9, 1968 Anthony N. Chirico It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 32, for "efficiently" read efficiency column 3, line 21, for "exect" read effect column 4, line 70, for "liquor." read liquor; same column 4,

after line 70, insert the following paragraph:

(e) passing part of the resulting concentrated liquor from said second effect directly to an evaporating chamber maintained under a subatmospheric pressure whereby vaporization of liquor and crystallization of the first solute occurs resulting in further concentration of said liquor with respect to the second solute, separating the crystals produced from concentrated liquor and recovering relatively pure concentrated liquor, and passing a slurry of said crystals from said second effect to a recovery means, separating said crystals from said slurry and returning the resulting mother liquor to the second effect.

Signed and sealed this 11th day of February 1969.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A METHOD FOR RECOVERING A FIRST SOLUTE FROM LIQUOR AND RECOVERING LIQUOR RELATIVELY CONCENTRATED WITH RESPECT TO A SECOND SOLUTE, COMPRISING: (A) INTRODUCING THE FEED LIQUID TO THE RECIRCULATION SYSTEM OF A FIRST EVAPORATOR CRYSTALLIZER EFFECT, AND HEATING SAID FEED LIQUOR BY INDIRECT HEAT EXCHANGE, AND PASSING THE RESULTING HEATED FEED LIQUOR TO SAID FIRST EFFECT, SAID FIRST EFFECT MAINTAINED UNDER A PRESSURE GREATER THAN ATMOSPHEREIC AND AT AN ELEVATED TEMPERATURE, WHEREBY VAPORIZATION OF THE LIQUOR AND CRYSTALLIZATION OF THE FIRST SOLUTE OCCURS RESULTING IN LIQUOR RELATIVELY MORE CONCENTRATED THAN SAID FEED LIQUOR WITH RESPECT TO THE SECOND SOLUTE; (B) PASSING STEAM THROUGH A THERMO-COMPRESSOR COMMUNICATING WITH SAID FIRST EFFECT TO ASPIRATE VAPORS GENERATED IN SAID FIRST EFFECT, AND RECIRCULATING A PORTION OF SAID VAPORS WITH SAID STEAM TO HEAT SAID FEED LIQUOR IN INDIRECT HEAT EXCHANGE IN SAIF FIRST MENTIONED HEAT EXCHANGER; (C) PASSING THE RESULTING CONCENTRATED LIQUOR FROM SAID FIRST EFFECT TO A SECOND EVAPORATOR CRYSTALLIZER EFFECT, SAID SECOND EFFECT MAINTAINED AT SUBATMOSPHERIC PRESSURE AND AT A TEMPERATURE RELATIVELY LOWER THAN SAID FIRST EFFECT AND HEATED AT LEAST IN PART FROM VAPORS FROM SAID FIRST EFFECT BY INDIRECT HEAT EXCHANGE, WHEREBY VAPORIZATION OF LIQUOR AND CRYSTALLIZATION OF THE FIRST SOLUTE OCCURS RESULTING IN FURTHER CONCENTRATION OF SAID LIQUOR WITH RESPECT TO THE SECOND SOLUTE; AND (D) SEPARATING THE CRYSTALS PRODUCED FROM SAID CONCENTRATED LIQUOR. 